Integrated circuit technology relies on transistors to formulate functional circuits. The complexity of these circuits requires the use of an ever-increasing number of transistors. As the number of transistors increases, the surface space on the semiconductor substrate becomes more valuable.
Miniaturization is the process of crowding more active devices upon a given amount of semiconductor surface space, while improving both fabrication cost and device performance. As the surface space on a semiconductor substrate becomes more crowded with active devices, one solution to the crowding is to build a device that is in a vertical orientation to the semiconductor substrate.
Semiconductor processing of metal oxide semiconductor field-effect transistors (MOSFETs) requires the formation of a gate that will make an active device. The gate may typically be a conductor that is insulated from the semiconductor substrate by a dielectric layer such as a gate oxide layer over the semiconductor substrate. One particular MOSFET of interest is the vertical MOSFET because the gate is disposed vertically downward into the semiconductor substrate. As miniaturization progresses, the size and shape of the vertical transistor and the quality of the gate oxide layer become more important to both processing yield and to field use life.
Typically, a vertical transistor is formed in a rectilinear recess that includes two substantially vertical sidewalls and a substantially horizontal bottom. The recess is next covered with a dielectric layer. As such, the corner between sidewall and bottom may be substantially a right angle or there about, as the sidewalls are sometimes tapered outward from bottom to top of the recess. Because of the corner, there exists a greater likelihood of the dielectric layer breaking down at the corner because of enhanced electrical field activity at the corner. Consequently, the corner causes a reliability issue for the transistor as a field failure will result once the dielectric layer has broken down.